Coverage holes are areas where clients can’t receive a signal from the wireless network. If clients on an AP are detected at low received signal strength indicator levels, Cisco lightweight APs send a coverage hole alarm to the cisco WCS/NCS or PI.

The RRM coverage hole detection algorithm can detect areas of radio coverage in a wireless LAN that are below the level needed for robust radio performance. This feature can alert us to the need for an additional (or relocation) lightweight access point.

If clients on a lightweight access point are detected at threshold levels lower than those specified in the RRM configuration, the access point sends a “coverage hole” alert to the controller. The alert indicates the existence of an area where clients are continually experiencing poor signal coverage, without having a viable access point to which to roam.

The controller uses the quality of client signal levels reported by the APs to determine if the power level of that AP needs to be increased. Coverage hole detection is controller independent, so the RF group leader is not involved in those calculations. The controller knows how many clients are associated with a particular AP and what the signal-to-noise ratio (SNR) values are for each client.

If a client SNR drops below the configured threshold value on the controller, the AP increases its power level to try to compensate for the client. The SNR threshold is based on the transmit power of the AP and the coverage profile settings on the controller.

The controller uses the following equation for detecting a coverage hole:

Depending on the number of clients that are at or below this value for longer than 60 seconds, coverage hole correction might be triggered, and the AP could increase its power level to try to remove the SNR violation.

If the AP is already at power level 1, it cannot increase the power any further, and clients at the edge of the cell coverage suffer a performance hit or disassociate altogether if the signal gets weak enough.

Aside from a real coverage hole, a client with a poor roaming logic might not roam to another AP as expected and be “sticky.” A sticky client can remain associated with an AP until the SNR is very low and triggers a false coverage hole detection.

The coverage hole algorithm also allows the network to heal itself if an AP fails. When a neighbor AP is lost, it increases the power of nearby APs as needed to compensate. Again, the increase in power for an AP is a gradual process, increasing the power one level at a time.

Data/Voice RSSI text box, enter the minimum receive signal strength indication (RSSI) (It must be between -60 to -90 dBm and can be different for voice and data) value for data/voice packets received by the access point. The value that we enter is used to identify coverage holes within our network.

Min Failed Client Count per AP text box, the minimum number of clients on an access point with an RSSI value at or below the data or voice RSSI threshold. The range can be from 1 to 75, and default value is 3.

Coverage Exception Level per AP text box, the percentage of clients on an access point that are experiencing a low signal level but cannot roam to another access point. The range is 0 to 100%, and default value is 25%.

Note: Coverage hole detection is no longer a global setting and can be enabled or disabled on a per-WLAN basis: Coverage hole detection is enabled by default on the WLAN. One of the reasons we might want to disable this is because if we know a device is going to roam, it is advised that we enable the wireless on the device so that it can assist in finding coverage holes. Conversely, if several devices are stationary and have wireless as a backup, it would be advisable to disable this because we know the devices are not going to move and will not be able to provide intelligent information to help the coverage hole detection algorithm with its calculations.

This is one of the features of RRM on WLC and in this post we will see and learn the option under TPC.

This algorithm is responsible for reducing the power level on the APs to reduce excessive cell overlap and co-channel interference. TPC uses the RSSI calculations for the neighbor APs, and it determines effective changes only if there are more than three neighbor APs.

The TPC algorithm runs every 10 minutes (600Secs). The RF group leader runs TPC on a per-radio, per-AP basis. Therefore, a power adjustment on 802.11b/g has no bearing on the 802.11a power level settings for the same AP.

The minimum requirement for TPC is that a single AP needs to be heard by at least three other APs at -70 dBm or greater. Therefore, we must have at least four APs total. The logic behind the lowering of the power levels is that the third loudest neighbor is heard at -70 dBm or lower after the change.

The final purpose of the algorithm is to make sure that the third-loudest neighbor AP is heard at a signal level lower than the configured threshold (by default its –70 dBm).

***Note: The TPC algorithm is only responsible for turning power levels down.

TCP goes through these stages which decide if a transmit power change is necessary:

Find out if there is a third neighbor, and if that third neighbor is above the transmit power control threshold (-70dBm).

Determine the transmit power using this equation:

Tx_Max for given AP + (Tx power control thresh – RSSI of 3rd highest neighbor above the threshold).

Compare the calculation from step two with the current Tx power level and verify if it exceeds the TPC hysteresis.

If Tx power needs to be turned down: TPC hysteresis of at least 6dBm must be met. OR

If Tx power needs to be increased: TPC hysteresis of 3dBm must be met.

***Note: When a brand new APs boot up for the first time, it transmit at their maximum power level (its 1). When AP is power cycled, it uses their previous power settings.

***Note: It is important to remember that decreases in AP radio power levels are gradual, whereas increases can take place immediately. Therefore, if we change the RRM configuration settings, do not expect to start seeing the APs changing channels and adjusting their power as soon as we click Apply.

Now we will see the configuration steps@TPC

Via GUI:

Go to Wireless->802.11a/nor 802.11b/g/n -> RRM->TPC

On this screen we have these options:

Power Level Assignment Method: There are 3 ways to configure TPC algorithm:

Automatic: This is the default configuration and the TPC algorithm runs every ten minutes (600 seconds).

On Demand: The algorithm can be manually triggered if we click the Invoke Channel Update Now

Fixed

Min/Max Power: Maximum and minimum power level assignment and we can choose between -10 to 30dBm.

Power Threshold: Default value for this parameter is –70 dBm but can be changed when access points are transmitting at higher (or lower) than desired power levels.

Power Neighbor Count: The minimum number of neighbors an AP must have for the TPC algorithm to run.

Power Assignment Leader: This field displays the IP address of the WLC that is currently the RF Group Leader. Because RF Grouping is performed per-AP, per-radio, this value can be different for the 802.11a & 802.11b/g networks.

Last Power Level Assignment: The TPC algorithm runs every 600 seconds (10 minutes). This field only indicates the time (in seconds) since the algorithm last.

In this post we will learn about DCA and it’s a really cool feature of RRM.

DCA is managed by RF Group Leader (How to define RF leader, we saw in one of my last post)

DCA used to determine the optimal AP channel based on these parameters.

Load: Percentage of time spent transmitting 802.11 frames

Noise: Measurement of non-802.11 signals on every serviced channel

Interference: Percentage of radio time used by neighbor 802.11 transmissions

Signal strength: Received signal strength indication (RSSI) measurement of the received neighbor messages

These values are then used by the Group Leader to determine if another channel schema will result in at least a bettering of the worst performing AP by 5dB (SNR) or in other words: Based on these metrics, if the worst performing AP will benefit by at least 5 dB or more, a channel change will take place. The decision to change the channel of an AP is also weighted to prevent a mass change within the RF group. We would not want to have a single AP change channel and have that change result in 20 other APs having to change their channel. The controller also takes into account how heavily an AP is used.A less utilized AP is more likely to have a channel change instead of a heavily used neighbor (isn’t it an interesting feature?). This helps mitigate client disassociations during a DCA event because a radio channel change disconnects all associated clients.

***Note: When an AP first boots up out the box, it transmits on channel 1 on the 802.11b/g radio and channel 36 for the 802.11a radio. The channels change according to any DCA adjustments if necessary. If a reboot occurs, the APs remain on the same channel they were using before the reboot until a DCA event occurs. If an AP is on channel 152 and reboots, it will continue to use channel 152 when it comes back up.

***Note: Radios using 40-MHz channels in the 2.4-GHz band or or 80MHz channels are not supported by DCA.

The RRM startup mode is invoked in the following conditions:

In a single-controller environment, the RRM startup mode is invoked after the controller is rebooted.

In a multiple-controller environment, the RRM startup mode is invoked after an RF Group leader is elected.

Configure DCA:

***We must disable 802.11a and b radio before changing the config. for DCA and then enable it again. Simplest way to enable/disable the radio is via CLI: